1. Field of the Invention
The present invention relates to an engine control apparatus for controlling a lean-burn air-fuel ratio of an air-fuel mixture in a lean-burn engine for a vehicle, and more particularly, to the engine control apparatus provided with an EGR (exhaust gas recirculation) system in which a small amount of exhaust gas is mixed with intake air, whereby reduction of nitrogen oxides in the exhaust gas and improvement of the running performance can be expected.
2. Description of the Related Art
Lean-burn engines have been studied and developed as fuel-saving engines for new-generation vehicles. In this type of the engine, swirl or turbulence is generated in a combustion chamber during air induction, and a leaner air-fuel mixture than that of a theoretical air-fuel ratio is burned. In such a lean-burn engine, the air-fuel mixture is so lean that the amount of exhausted HC and CO gases is originally small, whereas perfect combustion advances to increase the NOx exhaust gases. After a certain air-fuel ratio is reached, however, the NOx delivery decreases to improve the exhaust gas characteristics as the air-fuel ratio increases.
Prior art air-fuel ratio control techniques are described in Jpn. Pat. Appln. Laid-Open (KOKAI) Publication Nos. 60-27748 and 58-38354. In the case where the air-fuel ratio exceeds the lean-side limit, there is a possibility of a misfire, an increase of combustion fluctuation, and driving performance deterioration. In the conventional methods, torque variation is detected so that the air-fuel ratio is subjected to lean-limit control based on the detected value of the torque variation, whereby the misfire and lowering of the driving performance are prevented.
According to the techniques of the former publications, however, it cannot be determined whether or not the NOx is actually decreased while the engine is in an operating state, so that the air-fuel ratio must be inevitably decided to a set value within leaner side based on the characteristics of exhausted NOx concentration.
According to a technique described in Jpn. Pat. Appln. Laid-Open (KOKAI) Publication No. 58-13137, moreover, the NOx concentration is estimated indirectly from the cylinder pressure, and an EGR system and the like are controlled in accordance with the NOx concentration, whereby the NOx is decreased.
However, the air-fuel ratio control disclosed in the latter publication is adapted for the EGR system and the like, and cannot be applied to lean-burn air-fuel ratio control.
Conventionally, exhaust gas recirculation (EGR) control is widely used as an effective method for restraining the formation of nitrogen oxides at the time of combustion. In the EGR control, the amount of the exhaust gas is mixed with the intake air, thereby increasing heat capacity of the gases in the cylinder to lower the temperature of the burning gas relatively. If the EGR amount is too large, however, the combustion fluctuation is caused to lower the output, fuel cost performance, and reliability of running performance. It is generally known, therefore, that the EGR amount must be restricted to a necessarily minimum value.
According to a technique described in Jpn. Pat. Appln. KOKAI Publication No. 2-252958, for example, an EGR control region is divided into two parts, feedback and open-loop control regions, depending on the engine driving conditions. In the feedback control region, a target EGR rate for each driving condition is set by referring a map, using the accelerator pedal opening degree and engine speed, whereby the EGR rate is controlled.
In the EGR disclosed in the publication, however, the target EGR rate is set depending on the driving conditions. Although the EGR control can be effected corresponding to the quantity of NOx produced, therefore, it is impossible to restrain combustion fluctuation which accompanies the decrease of the exhaust of the NOx. Accordingly, satisfactory running performance cannot always be obtained in the EGR control region.
Described in Jpn. Pat. Appln. Laid-Open (KOKAI) Publication No. 2-298657, moreover, is a technique such that the EGR amount is controlled in accordance with the intensity of combustion light emitted in the combustion chamber, whereby a sudden increase of the combustion speed is avoided, and the generation of the NOx is restrained.
According to this method, the EGR control is carried out depending on the state of combustion which is detected by the light intensity. Since the NOx exhaust is not detected, however, the EGR rate is not accurately controlled.